System for nondestructively determining composite material parameters

ABSTRACT

A method and apparatus for nondestructively determining fiber volume fraction and resin porosity of a composite material constructed of at least two different constituent materials wherein the following parameters of the composite material to be tested are known: density, elastic moduli of the constituent materials and layup sequence. Two acoustic waves of different polarizations are propagated through the composite material and the acoustic waves propagated through the composite material are sensed and the velocity of each of the two acoustic waves, V 1  and V 2 , are determined. The thickness of the composite material is determined. The fiber volume fraction and resin porosity of the composite material are then determined using the velocities, V 1  and V 2 , the thickness and known parameters of density, elastic moduli of the constituent materials and layup sequence.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of co-pending U.S. patentapplication Ser. No. 07/371,653, filed 21 Jun. 1989, now U.S. Pat. No.5,031,457 which application is a Continuation of U.S. patent applicationSer. No. 07/309,004 filed 7 Feb. 1989, now abandoned, which is acontinuation of U.S. patent application Ser. No. 07/147,155, filed 22Jan. 1988, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to systems for determiningparameters of materials and, more particularly, but not by way oflimitation to systems for nondestructive determining fiber volumefraction and resin porosity of composite materials constructed of atleast two different constituent materials.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a method for non-destructivelydetermining fiber volume fraction and resin porosity of a compositematerial constructed of at least two different constituent materialswherein the following parameters of the composite material to be testedare known: density, elastic moduli of the constituent materials andlayup sequence, the method comprising the steps of: propagating twoacoustic waves of different polarizations through the compositematerial; receiving the acoustic waves propagated through the compositematerial; determining the velocity of each of the two acoustic wavespropagated through the composite material from the received acousticwaves propagated through the composite material, the respectivevelocities being V₁ and V₂ ; determining the thickness of the compositematerial; and determining the fiber volume fraction and resin porosityof the composite material using the velocities, V₁ and V₂, the thicknessand the known parameters of density, elastic moduli of the constituentmaterials and layup sequence. The present invention also provides anapparatus for nondestructively determining fiber volume fraction andresin porosity of a composite material constructed of at least twodifferent constituent materials wherein the following parameters of thecomposite material to be tested are known: density, elastic moduli ofthe constituent materials and layup sequence, the apparatus comprising:means for propagating two acoustic waves of different polarizationsthrough the composite material; means for receiving the acoustic wavespropagated through the composite material and outputting the receivedacoustic waves in a digital format; means for determining the thicknessof the composite material; and a processor receiving the two acousticwaves in a digital format and determining the velocity of each acousticwave, V₁ and V₂, the processor having inputted therein the thickness ofthe composite material and having inputted therein the known parametersof density, elastic moduli of the constituent materials and layupsequence, the processor determining the fiber volume fraction and resinporosity of the composite material using the velocities, V₁ and V₂, thethickness and the known parameters of density, elastic moduli of theconstituent materials and layup sequence.

In another aspect, the present invention provides an apparatus fornondestructively determining fiber volume fraction and resin porosity ofa composite material constructed of at least two different constituentmaterials wherein the following parameters of the composite material tobe tested are known: thickness, density, elastic moduli of theconstituent materials and layup sequence, the apparatus comprising:means for propagating two acoustic waves of different polarizationsthrough the composite material; means for receiving the acoustic wavespropagated through the composite material and outputting the receivedacoustic waves in a digital format; means for determining the velocityof each of the two acoustic waves propagated through the compositematerial from the received and sensed acoustic waves propagated throughthe composite material and, the respective velocities being V₁ and V₂ ;and a processor receiving the two acoustic waves in a digital format anddetermining the velocities, V₁ and V₂, of the respective acoustic wavespropagated through the composite material, the processor having inputtedtherein the known parameters of thickness, density, elastic moduli ofthe constituent materials and layup sequence, the processor determiningthe fiber volume fraction and resin porosity of the composite materialusing the velocities, V₁ and V₂, and the known parameters of thickness,density, elastic moduli of the constituent materials and layup sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating the geometry of a specimen ofcomposite material to be tested.

FIG. 2 is a chart illustrating the effects of fiber volume fraction andresin porosity on wave propagation for a typical graphite-epoxy laminatecomposite material.

FIG. 3A, 3B and 3C are a series of three diagrams illustrating aniterative search algorithm for determining fiber volume fraction andresin porosity.

FIG. 4 is a comparison of ultrasonic and quantitative image analysisdata.

FIG. 5 is a diagrammatic, schematic view of a system constructed inaccordance with the present invention for nondestructively determiningfiber volume fraction and resin porosity of composite materials.

FIG. 6 is another system which is constructed in accordance with thepresent invention for nondestructively determining fiber volume fractionand resin porosity of composite materials.

FIG. 7 is yet another system constructed in accordance with the presentinvention for nondestructively determining fiber volume fraction andresin porosity of composite materials.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In recent years the use of composite materials has increasedsignificantly. In such materials, two different constituent materialsare combined to optimize the properties of the resulting compositematerial. For example, high strength fibers are embedded in plasticmaterials to achieve a composite material which is light weight and hasa high strength or stiffness to weight ratio. As used herein the term"composite materials" means any material constructed of at least twodifferent constituent materials.

Inhomogeneities can develop during the processing stage when thelaminate (composite material) is cured to its final solid state.Unwanted gases may be introduced into the composite material from avariety of sources including entrainment during mixing, entrapment ofair between plies during layup and evolution of volatiles during thecuring reaction. In an attempt to keep porosity at a minimum, a porousbleed ply usually is placed in contact with the laminate (compositematerial). In the fabrication process, temperature is increased:initially to lower the resin viscosity for better void and resintransport and ultimately to promote the cure reaction. Simultaneously,pressure is applied to force the unwanted gases and excess resin fromthe composite into the bleed ply, which is discarded after fabrication.During the process, fibers also can shift position, resulting in areaswhich are relatively resin rich or resin poor. When the process worksproperly, the result is a void-free microstructure with a uniformdistribution of reinforcing fibers. When the process breaks down, weakareas with excess resin or porosity may be created.

The present invention provides a system for nondestructively determiningfiber volume fraction, the percent of volume occupied by one of theconstituent materials (fibers), and resin porosity, the percent ofvolume that is void or occupied by air, in composite materials.

In the present system, the following parameters of the compositematerial to be tested are taken as known: density, elastic moduli of theconstituent materials and layup sequence.

Wave Propagation

The equations of motion for a continuum are given by:

    ρu.sub.i =σ.sub.ij,j                             (1)

where

ρ=density

u=particle displacement

σ_(ij) =stress tensor components

and i signifies differentiation w.r.t. indicated subscript. By insertingthe stress-strain relationship for an anisotropic solid:

    σ.sub.ij =C.sub.ijkl ε.sub.kl                (2)

where

C_(ijkl) =elasticity tensor components

ε_(kl) =strain tensor components

into the equations of motion, Eq. (1) becomes:

    ρu.sub.i =C.sub.ijkl u.sub.k,lj                        (3)

Assuming a plane wave solution of the form

    u.sub.i =A.sub.σ α.sub.i e.sup.i (kl.x -wt)    (4)

where

w=frequency

k=wave number

l=wave normal

A.sub.σ =amplitude of particle displacements

α=displacement (direction cosines)

we obtain the following eigenvalue equation for the velocities ofultrasonic wave propagation in any direction (l₁, l₂, l₃) in ananisotropic material ##EQU1## The geometry of the problem is illustratedin FIG. 1, where the plane of fiber reinforcement has been chosen to bethe x₂ -x₃ plane. Since most composite applications are for plate typestructures, we are limited for most practical cases to wave propagationin the direction perpendicular to the reinforcing plane, i.e.,1=(1,0,0). For this case, the eigenvalue equation for an orthotropicmaterial assumes the form: ##EQU2## which yields three possible wavemotions: one pure mode longitudinal wave with velocity √C_(1111/)ρ andtwo pure mode shear waves with velocities √C_(1313/)ρ and √C_(1212/)ρ.For wave propagation in any anisotropic material, one must be concernedwith possible complications from energy flux deviations from the wavenormal. However, for pure mode longitudinal wave propagation, energy cannever deviate from the wave normal. Furthermore energy flux deviation isnot observed for pure mode shear wave propagation in a directionperpendicular to a plane of reflection symmetry. Therefore, if werestrict our attention to symmetric laminates (this encompassesvirtually all practical laminate stacking sequences), energy fluxdeviation may be safely neglected.

Material Properties

In order to assess the effects of resin porosity and fiber loading onultrasonic behavior, it is necessary to first consider the behavior of asingle ply in the context of micromechanics. Since voids will residecompletely in the matrix, we begin by using the expressions of Boucher,"On the Effective Moduli of Isotropic Two-Phase Elastic Composites". J.Composite Materials, Vol. 8, 1974, pp. 82-89, to modify the materialproperties of an ideal matrix to account for the presence of porosity.

The next step in the procedure is to determine the mechanical propertiesof each individual ply from the known properties of the reinforcingfibers and the calculated properties of the resin. This micromechanicsproblem has been the subject of extensive research using a variety ofdifferent approaches. Unfortunately, there are no exact solutionsavailable for the problem of a random array of cylindrical reinforcingfibers embedded in an isotropic matrix. However, several investigatorshave developed suitable expressions (bounds) for the elastic moduli offiber reinforced composites which can be used as approximations for thispurpose. In this investigation, the expressions developed by Hashin, "Onthe Elastic Behavior of Fiber Reinforced Materials of ArbitraryTransverse Phase Geometry," J. Mech. Phys. Solids, Vol. 13, 1965, pp.119-134, for the upper bounds on the pertinent moduli were used, basedon their accuracy and ease of implementation on a minicomputer.

Once the ply properties have been determined, it is then necessary tocombine the individual properties in an appropriate manner for theparticular stacking sequence to obtain the overall laminate properties.This is done using the equations of classical laminate theory.

The procedure for determining porosity and fiber volume fraction isoutlined below.

Ultrasonic Velocity Measurement

A variety of analog techniques are available for the precisedetermination of transit time for ultrasonic wave propagation, Truell,R., Elbaum, C., and Chick, B., Ultrasonic Methods in Solid StatePhysics, Academic Press, 1969. Any of these methods would be acceptablefor this purpose. However, the advent of high speed digital dataacquisition and processing techniques means that this process can beautomated. In this application we employ a technique developed by Egle,D., "Using the Acoustoelastic Effect to Measure Stress in Plates",UCDL-52914, Lawrence Livermore Laboratory (1980) to achieve this end.This process requires initially that the ultrasonic signals bedigitized. An autocorrelation algorithm is then employed to estimate thetransit time through the material. This estimate is then refined, usinga curve fitting technique to find the maximum in the autocorrelationfunction. This approach has been found to yield the necessary accuracyin transit time measurements (to within 1 nanosecond) for microstructurecharacterization.

Data Analysis

The effects of fiber volume fraction and resin porosity on wavepropagation for a typical graphite-epoxy laminate configuration areshown in FIG. 2. This figure is based on typical values for themechanical properties of the constituent materials as shown in Table 1.

                  TABLE I                                                         ______________________________________                                        Mechanical Properties of Composite                                            Constituent Materials                                                         Density      k          m        G                                            (gm/cc)      (GN/m)     (GN/m)   (GN/m)                                       ______________________________________                                        Resin   1.26      7.7       2       2                                         Graphite                                                                              1.77     14.9       5.5    24                                         ______________________________________                                    

The layups studied in this program include a unidirectional laminate(shown) two cross-ply laminates, two angle ply laminates, and aquasiisotropic laminate. It should be noted that the velocity approachto fiber fraction/porosity measurement is also applicable to othercomposite systems (fiberglass, Kevlar, metalmatrix, etc.). However,resolution capability may vary from system to system, depending upon therelative differences in material properties between the fiber andmatrix.

The algebraic complexity of the problem (see theoretical section) makesit relatively difficult to solve explicitly for even the simple case ofunidirectional reinforcement. For practical laminates, the situation iseven more complicated. Clearly, an alternative approach is needed.Ideally, this approach should be rapid, accurate, sufficiently flexibleto handle various composite systems and configurations, reliable andeasy implement on a commonly available device such as a personalcomputer. A computer code with these desired characteristics wasdeveloped.

An iterative search algorithm was devised. It is illustrated in FIG. 3.First the points corresponding to the measured velocities are locatedfor varying fiber volume fractions from 25% to 75% in 5% increments forthe coarse mesh and resin porosities 0% to 25% in 5% increments. Thedistance in velocity space is given by: ##EQU3## between each of themesh points of the coarse mesh and the point corresponding to themeasured velocities. The mesh point closest to the measured point isthen the one which minimizes the distance measure as defined above. Oncethis point is identified, it serves as the base point for a new meshwith finer increments (+1% in both porosity and fiber content) than thatof the original coarse mesh. The process is then repeated to identifythe nearest point among the elements of the second mesh to thatmeasured. Then, the entire process is repeated one last time with arelatively fine mesh (+0.1% increments) to establish the final solution.While further refinements are possible by repeating the processindefinitely, differences on this order have little physicalsignificance and do not justify the additional time which would berequired to further refine the calculation. The ability of the techniqueto resolve fine microstructural differences is also limited by the timeresolution capability of the pulseecho overlap technique. In this case,we were capable of measuring transit time differences of 1 nanosecond.

Experimental Verification

In order to assess the utility of this technique, ultrasonic testresults were compared with microscopic measurements of porosity andfiber volume fraction. Test samples were machined from a 24 ply, 30.5 cm×30.5 cm unidirectionally reinforced panel manufactured by Lear Fan.This material was fabricated from Fiberite hy-E 1048 prepreg tape usingstandard autoclave processing techniques.

Quantitative measurements of local fiber content and porosity and fibervolume fraction were made along the edges of selected samples using theultrasonic technique described previously. These measured samples weresectioned and mounted in epoxy for microscopic analysis. Specimens wereabrasively polished and placed in a microscope with quantitative imageanalysis capability (Quantamet) for automated measurement ofmicrostructural constituents. Results from a typical sample arepresented in FIG. 4. Good agreement, both qualitatively andquantitatively, was observed between the ultrasonic predictions of localfiber content and the microscopic measurements. Estimates of fiberloading from the two techniques were usually within 3% of each other.Both techniques predicted that there was negligible porosity present inthe samples (less than 1.5%). Since there is an inherent uncertainty of2% in the quantitative image analysis system, better agreement betweenthe two methods was not expected. It should also be pointed out that thetwo techniques are not measuring precisely the same quantity. Theultrasonic test is sensitive to material property variations in acylindrical volume (whose diameter is that of the transducer of 0.31 cmin this case). The image analysis approach measures changes averagedover a plane area (0.49 xm ×0.34 cm) rather than a volume. Given thelevel of material inhomogeneity observed in the velocity scans, somedifferences in the measurement techniques are to be expected.

Accordingly, it may be concluded that the results from the two methodsare in substantive agreement, at least within experimental error.

Conclusions

1. A novel method for measuring local fiber content and porosity incomposite materials has been developed.

2. The method is based upon a composite micromechanics model for theeffects of fiber content and resin porosity on mechanical properties.The computer code developed in this research effort requires onlyultrasonic velocity measurements for the microstructure determination.

3. The method is rapid, nondestructive, and applicable to virtually anycomposite material system with any stacking sequence.

4. Tests have been conducted on samples of a unidirectionallyreinforced, 24 ply graphite-epoxy laminate to examine the utility ofthis method. Comparison of the predictions of local fiber content andporosity from the ultrasonic data and quantitative image analysisindicated that the two techniques were in substantial agreement with oneanother with the differences observed attributable to experimentalerror.

Embodiment of FIG. 5

Shown in FIG. 5 is a system 10 which is constructed to nondestructivelydetermine fiber volume fraction and resin porosity of a compositematerial in accordance with the present invention and in accordance withthe technique described in detail before. The system 10 basicallyincludes: a support structure 12 for operatively supporting a sheartransducer 14, a longitudinal transducer 16 and a linearly variabledisplacement transducer 18; a multiplexer 20; a pulser-receiver 22; ananalog to digital converter 24 (designated A/D in FIG. 5); a processor26; a power supply and signal conditioner 28; and a digital voltmeter30. In one operational embodiment, the system 10 was constructedutilizing the following commercially available components:

    ______________________________________                                        a.    shear transducer 14                                                                              Panametrics,                                                                  Model V-155                                          b.    longitudinal transducer 16                                                                       Panametrics,                                                                  Model V-109                                          c.    linearly variable  Shaevitz,                                                  displacement transducer 18                                                                       Model PCA-220-005                                    d.    multiplexer 20     Sonotek, Inc.,                                                                Model 23HV                                           e.    pulser-transducer 22                                                                             Panametrics,                                                                  Model 5052                                           f.    analog to digital  Sonotek, Inc.,                                             converter 24       Model STR *825                                                                an associated                                                                 software                                             g.    processor 26       Zenith,                                                                       Model 248                                            h.    power supply and   Albia Electronics                                          signal conditioner 28                                                                            Model DM-6                                           i.    digital voltmeter 30                                                                             Hewlett Packard                                                               Model 3440                                           ______________________________________                                    

In the operational embodiment just described, the particular analog todigital converter 24, Sonotek, Inc. STR *825, plugs directly into theprocessor 26 and the software associated with this particular analog todigital converter 24 is operatively disposed in the processor 26 foroperating the analog to digital converter 24.

The pulser receiver 22 is constructed and adapted to output timedexcitation pulses over a signal path 32 to either the longitudinaltransducer 16 or shear transducer 14. The pulser receiver also serves toamplify the sensed acoustic waves by the two transducers. Themultiplexer 20 allows the operator to automatically switch between thelongitudinal transducer 16 or shear transducer 14 as needed.

The excitation pulses are received by the shear transducer 14 orlongitudinal transducer 16 and the transducers are constructed to causean ultrasonic wave to propagate in the test sample, in response toreceiving such excitation pulses.

The shear transducer 14 and the longitudinal transducer 16 each also areconstructed to receive ultrasonic or acoustic waves propagated throughthe composite material being tested and to output in an analog formatsuch received waves. The shear transducer 14 outputs received wavespropagated through the composite material to be tested over a signalpath 38. The longitudinal transducer 18 outputs in an analog formatreceived waves propagated through the composite material to be testedover a signal path 40.

The waves propagated through the composite material are sensed bytransducers and outputted by the shear transducer 14 and thelongitudinal transducer 16 over the respective signal paths 38 and 40.These analog signals are inputted into and received by the multiplexer20. The multiplexer 20 outputs the waves propagated through thecomposite material and received from the shear transducer 14 over asignal path 42 which are inputted into the pulser receiver 22. Themultiplexer 20 also outputs the waves propagated through the compositematerial and received from the longitudinal transducer 16 over a signalpath 44 which are inputted into the pulser receiver 22. The pulserreceiver 22 outputs the waves propagated through the composite materialin an analog format received from the shear transducer 14 over a signalpath 46 which is inputted into the analog to digital converter 24. Thepulser receiver 22 also outputs the waves propagated through thecomposite material in an analog format over a common signal path 48which is inputted into the analog to digital converter 24.

The analog to digital converter 24 digitizes the received wavesoutputted by the shear transducer 14 and the analog to digital converter24 outputs the digitized waves over a signal path 50. The analog todigital converter digitizes the waves outputted by the longitudinaltransducer 16 and outputs the waves in a digital format over a signalpath 52. The digitized waves outputted by the shear transducer 14 andthe longitudinal transducer 16 respectively are inputted into theprocessor 26 by way of the respective signal paths 50 and 52.

The linearly variable displacement transducer 18 is connected to andreceives power from the power supply and signal conditioner 28 over asignal path 54. The linearly variable displacement transducer 18 has acontact end 56 on a plunger 58 which is spring loaded and mounted withinthe casing of the linearly variable displacement transducer 18. Thelinearly variable displacement transducer 18 is constructed to output adc voltage proportional to the displacement of the plunger 58 over asignal path 61 which is inputted into the digital voltmeter 40. Thedigital voltmeter 40 is constructed and adapted to provide a visuallyperceivable output indication of the voltage of the signal on the signalpath 61 which is proportional to the displacement of the plunger 58 inthe linearly variable displacement transducer 18 or, in other words,which is proportional to the thickness of the composite material as willbe made more apparent below.

The shear transducer 14, the longitudinal transducer 16 and the linearlyvariable displacement transducer 18 are operatively mounted on thesupport structure 12. More particularly, the shear transducer 14 isdisposed through an opening in a cylindrically shaped support plate 61and the upper end of the shear transducer 14 is supported a distanceabove the upper surface of the support plate 60 by way of a spring 62which is disposed about the shear transducer 14 and biases the sheartransducer 14 in an upward direction. The longitudinal transducer 16 isdisposed through an opening in the support plate 60 and the upper end ofthe longitudinal transducer 16 is support distance above the surface ofthe support plate 60 by way of a spring 64 which biases the longitudinaltransducer 16 in an upwardly direction. The linearly variabledisplacement transducer 18 is disposed through an opening in the supportplate 60 and the upper end of the linearly variable displacementtransducer is supported a distance above the upper surface of thesupport plate 60 by way of a spring 66 which is disposed about thelinearly variable displacement transducer 18.

The support plate 60 is disposed generally below an arm 68. A shaft 70is disposed through a portion of the arm 68. One end of the shaft 70 issecured to a central portion of the support plate 60. The opposite endof the shaft 70 extends through a knob 72 and the shaft 70 is secured tothe knob 72. The shaft 70 is connected to the support plate 60 and theknob 72 so that, by manually rotating the knob 72, the support plate 60is rotated.

An actuator post 74 is disposed through an opening in one end of the arm68 and the actuator post 74 is supported within this opening by way of aspring 76 so that one end of the actuator post 74 is supported adistance above the upper surface of the arm 68 by way of the spring 76.The actuator post 74 is supported on the arm 68 so that an actuating end78 of the actuator post 74 is supported and disposed a distance abovethe upper surface of the support plate 60 in a nonactuated position ofthe actuator post 74. A set screw 80 is disposed through a portion ofthe arm 68 and one end of the set screw 80 is positioned to contact aportion of the actuator post 74 in one position of the set screw 80 tosecure the actuator post 74 or the transducers 14 or 16 in apredetermined position within the respective openings in the arm 68during the operation of the system 10 in a manner and for reasons whichwill be made more apparent below.

One end of the arm 68 is secured to one end of a support rod 82 and theopposite end of the support rod 82 is secured to a base 84. The supportrod 82 cooperates to support the arm 68 and the support plate 60 adistance above an upper surface 86 of the base 84. The support rod 82also is sized so that the shear transducer 14, the longitudinaltransducer 16 and the linearly variable displacement transducer 18 eachare supported a predetermined distance above the upper surface 86 of thebase 84.

In operation, the composite material to be tested is placed on the uppersurface 86 of the base 84 in a position generally under the plunger 58contact end 56 of the linearly variable displacement transducer 18. Theactuator post 74 is then pressed manually against the bias action of thespring 76 thereby moving the actuating end 78 into engagement with theupper end of the linearly variable displacement transducer 18. Theactuator pulse 70 is manually moved in the downward direction until theupper end of the linearly variable displacement transducer 18 engagesthe upper surface of the support plate 60. The actuator post 74 can besecured in this position by the set screw 80 if desired. In thisposition of the linearly variable displacement transducer 18, the DCvoltage outputted by the linearly variable displacement transducer 18over the signal path 61 is proportional to the thickness of thecomposite material being tested and this voltage proportional tothickness is outputted over the signal path 61 and inputted into thedigital voltmeter 40. The digital voltmeter 40 provides a visuallyperceivable output indication indicating the DC volt of the signalinputted on the signal path 61, this voltage being proportional to thethickness of the composite material. The operator manually inputs thethickness of the composite material into the processor 26 using theprocessor 26 keyboard.

It should be noted that, in a more automated form or if desired in aparticularly application, the linearly variable displacement transducer18 output signal proportional to the thickness of the material can beinputted directly into the processor 26 thereby eliminating the manuallysteps of reading the digital voltmeter 40 in manually inputting thethickness into the processor 26 if desired.

The actuator post 74 then is released and moved by the spring 76 to therest position shown in FIG. 5 thereby causing the linearly variabledisplacement transducer 18 to be moved to the rest position by thespring 66. After the linearly variable displacement transducer 18 hasbeen moved to the rest position, the operator then rotates the knob 72thereby rotating the support plate 60 to position the upper end of thelongitudinal transducer 16 generally under the actuating end 78 of theactuator post 74. The actuator post 74 then is moved downwardly againstthe bias action of the spring 76 with the actuating end 78 thereofengaging and moving the longitudinal transducer 16 in the downwardlydirection. The longitudinal transducer 16 is moved in the downwardlydirection by the actuator post 74 until the lower end of thelongitudinal transducer 16 engages the upper surface of the compositematerial. The actuator post 74 is secured in this position by the setscrew 80 thereby securing the longitudinal transducer 16 in theoperating position wherein the lower end of the longitudinal transducer16 engages the upper surface of the composite material.

It should be noted that, prior to moving the longitudinal transducer 16to the operating position just described, a high viscosity couplingagent is applied to the lower end of the longitudinal transducer 16 forcoupling the ultrasonic vibrations of the longitudinal transducer 16 tothe composite material to be tested. Also, the high viscosity couplingagent is applied to a portion of the upper surface of the compositematerial to be tested. One coupling agent suitable for this purpose is aresin made by Dow Chemical, Model V9.

After the longitudinal transducer 16 has been moved to the operatingposition, the pulser receiver 22 is actuated to output excitation pulseswhich are multiplexed through the multiplexer 20 and inputted into thelongitudinal transducer 16 by way of the signal path 34. Thelongitudinal transducer 16 is constructed to vibrate in a particularmanner so that longitudinal ultrasonic waves are coupled to andpropagated through the composite material. The ultrasonic waves inducedby the longitudinal transducer 16 and propagated through the compositematerial to be tested propagate through the composite material and arereflected back through the composite material (back surface reflectionsin the particular embodiment of the invention shown in FIG. 5). Theultrasonic waves propagated through the composite material and reflectedback through the composite material are sensed and received by thelongitudinal transducer 16, and the longitudinal transducer 16 outputsan analog signal on the signal path 40 in response to receiving theultrasonic waves propagated through the composite material. Thesereceived signals are outputted on the signal path 40 in an analogformat. The signals outputted by the longitudinal transducer 16 on thesignal path 40 are indicative of a second velocity (V₂), the velocity ofthe ultrasonic wave propagated through the composite material emanatingfrom the longitudinal transducer 16.

The transducers 14, 16 and 18 must be positioned in the same positionduring the operation of the system 10 so the thickness measurement istaken and the ultrasonic waves are propagated through substantially thesame point on the composite material. Index marks could be inscribed onthe upper surface of the support plate 60 for alignment with one edge ofthe arm 68 to visually align each of the transducers 14, 16 and 18. Inthe alternative, index holes can be formed in the upper surface of thesupport plate 60 and a ball can be located in an opening in the lowersurface of the arm 68 with the ball being biased toward the arm 68 byway of a spring. Thus, when the support plate 60 is rotated, the ballfalls into one of the index holes in the support plate 60 to indicatethat the support plate 60 has been rotated to a correct position.

The signals outputted by the longitudinal transducer 16 are multiplexedthrough the multiplexer 20 and inputted into the pulser receiver 22 byway of the signal path 44. The pulser receiver outputs such signals on asignal path 46 for reception by the analog to digital converter 24. Theanalog to digital converter 24 receives the signals in the analog formatoutputted by the longitudinal transducer 16 and the analog to digitalconverter 24, operated by the processor 26 in accordance with theprogram mentioned before for operating the analog to digital converter24, digitizes the analog signals outputted by the longitudinaltransducer 16. The analog digital converter 24 outputs in a digitalformat the longitudinal transducer 16 output signals on the signal path52 which are inputted into the processor 26. The processor 26 isprogrammed to determined the second velocity (V₂) in response toreceiving the inputted transducer 16 output signals in the digitalformat from the analog to digital converter 24.

The actuator post 74 then is released and moved by the spring 76 to therest position shown in FIG. 5 thereby causing the longitudinaltransducer 16 to be moved to the rest position by the spring 64. Afterthe longitudinal transducer 16 has been moved to the rest position, theoperator then rotates the knob 72 thereby rotating the support plate 60to position the upper end of the shear transducer 14 generally under theactuating end 78 of the actuator post 74. The actuator post 74 then ismoved downwardly against the bias action of the spring 76 with theactuating end 78 thereof engaging and moving the shear transducer 14 inthe downwardly direction. The shear transducer 14 is moved in thedownwardly direction by the actuator post 74 until the lower end of theshear transducer 14 engages the upper surface of the composite material.The actuator post 74 is secured in this position by the set screw 80thereby securing the shear transducer 14 in the operating positionwherein the lower end of the shear transducer 14 engages the uppersurface of the composite material.

It should be noted that, prior to moving the shear transducer 14 to theoperating position just described, a high viscosity coupling agent isapplied to the lower end of the shear transducer 14 for coupling theultrasonic vibrations of the shear transducer 14 to the compositematerial. One coupling agent suitable for this purpose is a resin madeby Dow Chemical, Model V9, as mentioned before.

After the shear transducer 14 has been moved to the operating position,the pulser receiver 22 is actuated to output excitation pulses which aremultiplexed through the multiplexer 20 and inputted into the sheartransducer 14 by way of the signal path 34. The shear transducer 14 isconstructed to vibrate in a particular manner so that shear ultrasonicwaves of a known polarization are coupled to and propagated through thecomposite material. The ultrasonic waves induced by the shear transducer14 and propagated through the composite material to be tested propagatethrough the composite material and are reflected back through thecomposite material (back surface reflection in the particular embodimentof the invention shown in FIG. 5). The ultrasonic waves propagatedthrough the composite material and reflected back through the compositematerial are sensed and received by the shear transducer 14. The sheartransducer 14 outputs an analog signal on the signal path 40 in responseto receiving the ultrasonic waves propagated through the compositematerial, these received signals being outputted on the signal path 40in an analog format. The signals outputted by the shear transducer 14 onthe signal path 40 are indicative of the velocities of the twoultrasonic waves ((V₁ and V₂) propagated through the composite materialby the shear transducer 14.

The signals outputted by the shear transducer 14 are multiplexed throughthe multiplexer 20 and inputted into the pulser receiver 22 by way ofthe signal path 44. The pulser receiver outputs such signals on a signalpath 46 for reception by the analog to digital converter 24. The analogto digital converter receives the signals in the analog format outputtedby the shear transducer 14 and the analog to digital converter 24,operated by the processor 26 in accordance with the program mentionedbefore for operating the analog to digital converter 24, digitizes theanalog signals outputted by the shear transducer 14. The analog todigital converter 24 outputs in a digital format the shear transducer 14output signals on a signal path 52 which are inputted into the processor26. The processor 26 is programmed to determined the first velocity (V₁)(the velocity of the factor of the two shear waves) in response toreceiving the inputted transducer 16 output signals in a digital formatfrom the analog to digital converter 24.

Prior to starting the operation of the system 10, the operator manuallyhas inputted into the processor 26 certain parameters of the compositematerial to be tested, namely, density, elastic moduli of theconstituent materials and layup sequence, which are stored in theprocessor 26. The processor 26 previously has the thickness of thecomposite material also has been inputted into the processor 26 in themanner described before. The processor 26 is programmed to store theinputted thickness of the composite material to be tested. After theprocessor 26 has determined the first and second velocities, V₁ and V₂,the processor 26 then is programmed to determined the fiber volumefraction and resin porosity of the composite material based on thedetermined parameters of thickness and first and second velocities, V₁and V₂, and the inputted known parameters of density, elastic moduli ofthe constituent materials and layup sequence in accordance with theprocedures graphically shown in FIGS. 2 and 3 and described before. Inone particular embodiment, the processor 26 was programmed with thefollowing program to enable the processor 26 to determine the fibervolume fraction and resin porosity of the composite material to betested in the manner just described, the program being outlined below inthe FORTRAN and C (subroutine PLT₋₋ Time) languages: ##SPC1##

The signals outputted by the longitudinal transducer 16 and the sheartransducer 14 comprise a first series of pulses generally referred to inthe art as the "main bang" followed after a time delay by another seriesof pulses referred to generally in the art as "first back surfacereflections", followed after a time delay by another series of pulsescommonly referred to in the art as "second back surface reflections".This sequence of a series of pulses followed by a time delay and thenanother series of pulses is repeated. The analog to digital converter 24digitizes this signal. The processor 26 could then be programmed todetermine the velocity (V₁) or (V₂) by determining the time delaybetween corresponding peaks of the first and the second back surfacereflections. However, in accordance with the program described above andin accordance with one mode of operating the present invention, theprocessor 26 is programmed to determine this time delay and thusdetermine the first and the second velocities, (V₁) and (V₂), using aquadratic fit to find the maximum in the autocorrelation function todetermine the peak in each of the first and the back surface reflectionsand the time delay between these two peaks then is utilized to determinethe respective velocities, (V₁) and (V₂).

It also should be noted that the particular analog to digital converter24 describe before digitizes at a rate of 25 MKz. This rate ofdigitizing does not provide the accuracy desired in most applications ofthe system 10. Thus, the program mentioned before in connection with theparticular analog to digital converter 24 cooperates the analog todigital converter 24 to artificially induce a higher accuracy byshifting the point in the analog signal to be digitized eight timesthereby providing an effective digitizing rate or sample rate of 200 MHzin this particular example.

Rather than using the autocorrelation function system for determiningthe peaks for the purpose of measuring the velocities, (V₁) and (V₂),the processor 26 could be programmed to simply measure or determine thetime delay between the peaks of the digitized signal. This is not donein the particular embodiment described before because this has beenfound not to be as accurate as the method previously described becauseof dispersion and attenuation phenomena. However, if the analog todigital converter 24 could be operated at a higher sampling ordigitizing rate, this peak to peak method could be utilized for higherdata acquisition.

With the particular model shown in FIG. 5 and particularly with thespecific embodiment shown described before, the outputs of the sheartransducer 14 and the longitudinal transducer 16 are displayed by theprocessor 26 so the operator can determine whether or not the receivedsignals are adequate for processing in accordance with the presentinvention. The operator is observing these signals on the processor 26display to ascertain whether or not the amplitudes are high enough or,in other words, whether or not this is a detectable signal. Theprocessor 26 in some applications could be programmed to make thisdetermination automatically.

As specifically described before, the system 10 utilizes a sheartransducer 14 and a longitudinal transducer 16. In the shear transducer14, vibrations are generated in response to the received excitationpulses and these vibrations result in the first ultrasonic wave beingemitted from the shear transducer 14 and propagated through thecomposite material to be tested. Assuming "l" equals a vector describingthe direction of propagation of the ultrasonic wave normal, and "α"equals a vector describing the direction of particle vibration in thecomposite material, then the ultrasonic wave propagated through thecomposite material as a result of a shear transducer 14 represent acircumstance where "l" is perpendicular to "α".

As specifically described before, the system 10 also utilizes alongitudinal transducer 16. In the longitudinal transducer 16,vibrations are generated in response to the received excitation pulsesand these vibrations result in the second ultrasonic wave being emittedfrom the longitudinal shear transducer 16 and propagated through thecomposite material to be tested in this circumstance, "l" is parallel to"α". Assuming "l" equals a vector describing the direction ofpropagation of the ultrasonic wave normal, and "α" equals a vector.

Thus, the waves propagated through the composite material as induced bythe shear transducer 14 and the longitudinal transducer 16 havedifferent polarizations, one instance, being were "l" is parallel to "α"in the case of the longitudinal transducer 16 and the other being were"l" is perpendicular to "α" in the case of the shear transducer 14. Inthe present invention, it only is important that two acoustic waves arepropagated through the composite material to be tested having differentpolarizations and the present application is not limited to theparticular polarizations described before with respect to thelongitudinal and the shear transducers 16 and 14.

It also should be noted that two waves having different polarizationscan be caused to be propagated through the composite material to betested using only a single shear transducer. In this instance, thecomposite material to be tested is placed in one position under thesingle shear transducer for inducing the first ultrasonic wave to bepropagated through the composite material. The composite material thenis moved and repositioned under the shear transducer for propagating thesecond ultrasonic wave through the composite material. If the compositematerial is moved in a proper manner to different positions as justdescribed, two waves having different polarizations can be induced inthe composite material using the signal shear transducer.

The specific program described before is particularly adapted forcomposite materials wherein the fiber constituent is disposed in theother material constituent in a two dimensional pattern. The presentinvention also could be utilized for three dimensional patterns;however, the processor 26 program would have to be modified toaccommodate such three dimensional patterns. In general, the programwould have to be modified in the following manner to accommodate threedimensional patterns for woven reinforcements, or for carbon-carbonmaterials.

EMBODIMENT OF FIG. 6

Shown in FIG. 6 is a system 10a which also is constructed in accordancewith the present invention for nondestructively determining fiber volumefraction and resin porosity of a composite materials. The system 10agenerally comprises a modified support structure 12a which is disposedin a reservoir 90 containing water, the support structure 12a beingimmersed in the water.

The support structure 12a is connected to cross beams 92 and 94 whichare supported on the upper end of the reservoir 90. In this embodiment,the composite material to be tested also is immersed within thereservoir as generally illustrated diagrammatically in FIG. 6 whereinthe composite material is designated by the reference numeral 96.

The support structure 12a includes a modified support plate 60a which isrollingly connected to the cross beams 92 and 94 by way of a supportbeam 98, one end of the support beam 98 being secured to the supportplate 60a and the opposite end of the support beam 98 being rollinglyconnected to the cross beams 92 and 94 so the support plate 60a can bemoved along the support beam 92 and alternatively along the support beam94 for positioning the support plate 92 in various positions within thereservoir 90 and with respect to the composite material 96.

In this embodiment, four longitudinal transducers are used. One pair(100 and 104) are employed at normal incidence for longitudinal wavepropagation. A second pair (14 and 16) are employed at an incidenceangle other than 90° and are used to generate shear waves via modeconversion. In this way, transducers 100 and 104 can be used forthickness measurement (since they are a known distance apart and thesound velocity in water is constant) as well as for longitudinal wavepropagation.

One end of a curved arm 102 is connected to the support plate 60a. Thearm 102 extends a distance from the support plate 60a so the oppositeend of the arm 102 is positioned generally below and spaced a distancefrom the first thickness transducer 100. A second thickness transducer104 is supported in the end of the arm 102, opposite the end connectedto the support plate 60a, so that the second thickness transducer 104 isaligned with the first thickness transducer 100. The second thicknesstransducer 104 also is constructed and operates exactly like thelongitudinal transducer 16 described before.

The transducers 14, 16, 100 and 104 each are connected through themultiplexer 20, the pulser receiver 22, the analog to digital converter24 and the processor 26 in a manner exactly like that described beforewith respect to the transducers 14 and 16.

In operation, the composite material to be tested is disposed in theimmersion bath within the reservoir 90. The support structure 12a ispositioned on the cross beam 92 or the cross beam 94 so that transducer100 is positioned generally above a point on the composite material 96and transducer 104 is positioned generally on the opposite side of thecomposite material 96 and aligned with the first thickness transducer100. In this position, the transducer 14 is angularly disposed withinthe support plate 60a so that the ultrasonic wave emitted by thetransducer 14 impinges on the point immediately below the firstthickness transducer 100 and generally between the first and the secondthickness transducers 100 and 104. In this position, transducer 16 isangularly disposed within the support plate 60 so that the ultrasonicwaves generated in the part by transducer 14 will be received bytransducer 16.

Transducer 14 and transducer 16 are operated in a pitch-catch moderather than the pulse-echo mode described in FIG. 5. Otherwise, they areused to provide shear velocity information analogous to that provided bythe single contact shear transducer shown in FIG. 5 (14).

Transducer pair 100 and 104 are used for two purposes. The first purposeis to determine the thickness of the composite sample. For this purpose,transducer 100 is excited by the pulser receiver 22 to generate alongitudinal wave in the water. This wave propagates to the uppersurface of the composite material where part of the energy is reflectedback to transducer 100. The reflected wave and successive reflectionsare sensed by this transducer. Since the velocity of sound wavepropagation in water is a known constant (1,460 m/s), by digitizing theresponse of transducer to the first two water path reflections, thedistance between transducer 100 can be determined. In a similar fashion,by exciting transducer 104 and digitizing the same two water pathechoes, its position relative to the lower surface of the composite isdetermined. Since the total distance between the surface of transducer100 and transducer 104 is fixed, this procedure yields the thickness ofthe composite.

The second purpose of the transducer pair is to determine the velocityof longitudinal wave propagation in a direction perpendicular to theplane of reinforcement. The device may be operated in a pulse-echo modewith a single transducer (either 100 or 104) serving as generator andreceiver or with one transducer serving as generator and the other asreceiver. By now analyzing successive internal reflections within thecomposite, the transit time for this digital mode can be measured. This,in conjunction with the thickness measurement, yields the desiredlongitudinal velocity (V₂).

In this operation of system 10a, the first and second velocities aredetermined in a manner similar to that described with respect to system10 and the processor is programmed to calculate resin porosity and fibervolume fraction as in system 10. The processor is also programmed totranslate the transducer assembly over the surface of the part so thatresin porosities and fiber volume fraction measurements can be performedfor the entire part.

EMBODIMENT OF FIG. 7

Shown in FIG. 7 is another modified system 10c which is constructed inaccordance with the present invention for nondestructively determiningfiber volume fraction and resin porosity of a composite material, thecomposite material being diagrammatically shown in FIG. 7 and designatedtherein by the reference numeral 106. In this system 10c, the transducer14 and the transducer 16 each are supported in a support plate (notshown) in a manner exactly like described before with respect to thesystem 10a shown in FIG. 6. Further, transducer 100 is supported in thesupport plate in a manner exactly like described before with respect tothe system 10a shown in FIG. 6. Transducer 104 is supported so thesecond thickness transducer 104 is disposed and oriented with respect tothe first thickness transducer 100 in a manner exactly like thatdescribed before with respect to the system 10a shown in FIG. 6.

In system 10c, a pair of water jets 112 are associated with each of thetransducers 14, 16, 100 and 104. The water jets associated with thetransducers 14, 16, 100 and 104 are schematically shown and representedin FIG. 7 by a pair of arrows associated with each transducer anddesignated by the reference numerals 110 and 112. The water jetsassociated with the transducer 14 are designated as 110d and 112d inFIG. 7, the water jets associated with the transducer 100 are designated110e and 112e in FIG. 7, the water jets associated with the transducer16 are designated 110f and 112f in FIG. 7 and the water jets associatedwith the transducer 104 are designated 110s and 112s in FIG. 7. Thewater jets 110 and 112 associate with each of the transducers areoriented to supply a jet of water generally between the end of thetransducer and the surface of the composite material 106.

The water jets or streams provided by the water jets 110 and 112 providea coupling for coupling the ultrasonic waves between the transducers andthe composite material 10 thereby eliminating the need for immersing thetransducers and the composite material in a reservoir containing thecoupling agent as described before in connection with FIG. 6.

In this embodiment of the invention, the transducers 14, 16, 100 and 104can be moved freely about the composite material 106. This embodiment ofthe invention permits the testing of large parts which are incapable ofbeing immersed practically in a reservoir.

In this embodiment of the invention, the fiber volume fraction and resinporosity of the composite material are determined by the processor 26 ina manner exactly like that described before with respect to the system10a shown in FIG. 6.

The procedures prescribed in detail before assume the state of cure ofthe composition material is known and therefore the elastic modulii ofthe constituent materials also are known. The elastic moduli of theconstituent materials of the composite material vary with the state ofcure and, where the state of cure is not known, procedures must beeffected to measure the effect of the cure reaction in the compositematerial. Preferably, the procedures are compatible with ultrasonicsound measurements. In general, dielectric property measurements(conductivity, capacitance, permitivity, loss factor) can be used toaccomplish this purpose.

Since ionic mobility is directly related to the extent ofpolymerization, measurement of conductivity can be correlated to thedegree of cure. Hence, mechanical properties (Young's modulus, shearmodulus, Poisson's ration, viscosity, etc.). It should be pointed outthat these correlations are empirical and must be done for each resinsystem under consideration. See Marvin Bramm Berg, David Day, Huan Leeand Kimberly Russell "New Applications For Dielectric Monitoring andControl In Advanced Composites: The Latest Developments", Published byAmerican Society of Metals (1986, pages 307-311). These relationshipsare the basis for the present method of compensating for localvariations in the extent of cure. It should also be mentioned that lowfrequency probes must be used for accurate cure measurements.

Initially, what is sometimes referred to herein as a "state of cure database" is accumulated. Sample composite materials, each having the sameconstituent material as the composite material to be tested and eachhaving a known state of cure, initially are established. Each of thesesample composite materials is cured to various known states of cure,such as ten percent (10%) cured, twenty percent (20%) cured . . . onehundred percent (100%) cured, for example. The elastic moduli (shear andYoung's moduli for example) and the dielectric property (e.g.conductivity) of each of these resin samples then is determined.

From this set of experimental or empirical data, curves plotting twoindependent elastic moduli such as shear and Young's versus conductivityfor each of the sample composite materials are developed. These curvescomprise the state of cure data base which is inputted into theprocessor 26. In a more preferred form, a formula is developedrepresenting each of these curves and these formulae comprise the stateof cure data base which is inputted into the processor 26. In eithercase, the state of cure data base is inputted into and stored in theprocessor 26.

The dielectric property measuring device outputs a signal which isindicative of the conductivity of the composite material to be tested.

Dielectric property devices which are capable of providing an outputproportional to the capacitance of the composite material to be testedare well known in the art and one such device which can be used in thepresent invention is commercially available from Micromet Instruments,Inc , Eumetric System II

The dielectric constant indicating device outputs an indication of thedielectric constant of the composite material being tested, and thisoutput is digitized and inputted into the processor 26. Where thedielectric constant indicating device more particularly outputs anindication of the conductivity of the composite material to be tested,the processor 26 is programmed to determine the dielectric constant ofthe material to be tested from the inputted indication of measuredconductivity.

The processor 26 has stored therein the state of cure data base, and theprocessor 26 is programmed to determine the shear modulus and theYoung's modulus from the inputted output of the dielectric constantindicating device either from the curves stored in the state of curedata base or the formula stored in the state of cure data base.

After determining the required elastic modulii, the processor 26 thendetermines the other composite material parameters in the mannerdescribed before, but using the shear modulus and the Young's modulusdetermined in the manner just described.

With respect to the device as shown in FIGS. 6 and 7, the dielectricconstant indicating device cannot be incorporated with the othertransducers since water is used as the transmitting medium In thesecases, the dielectric constant indicating device which provides anoutput of the capacitance of the composite material being tested cannotbe used in a water medium In these instances where water is used as atransmitting medium, the dielectric constant indicating deviceseparately is utilized to determine the dielectric constant of thecomposite material being tested.

Further, with respect particularly to the embodiment shown in FIGS. 6and 7, it should be noted that the state of cure may, and in manyinstances will, vary over the area of composite material being testedFor example only, the state of cure along the edges of the compositematerial may be quite different as compared to the state of cure overthe central portion of the composite material to be tested. Thedielectric property of the composite material to be tested is determinedat a plurality of points over the entire area of the composite materialto be tested and the determined dielectric property are correlated withthe other measured parameters for determining the parameters inaccordance with the present invention.

Changes may be made in the construction and the operation of the variouscomponents and assemblies described herein and changes may be made inthe steps or the sequence of steps of the methods described hereinwithout departing from the spirit and the scope of the invention asdefined in the following claims.

I claim:
 1. A method, using a processor, for nondestructivelydetermining fiber volume fraction and resin porosity of a compositematerial constructed of at least two different materials wherein thefollowing parameters of the composite materials to be tested are known:density and layup sequence, the method comprising:determining theelastic moduli of the composite material to be tested and inputting theelastic moduli into the processor; propagating two independent acousticwaves through the composite material; receiving the acoustic wavespropagated through the composite material in the processor; determining,in the processor, the velocity of each of the two acoustic waves,propagated through the composite material from the received acousticwaves propagated through the composite material, the respectivevelocities being V₁ and V₂ ; determining the thickness of the compositematerial and inputting the thickness into the processor; anddetermining, in the processor, the fiber volume fraction and resinporosity of the composite material using the determined elastic moduli,the velocities, V₁ and V₂, the thickness, and the known parameters ofdensity and layup sequence.
 2. The method of claim 1 wherein the step ofdetermining the elastic moduli is defined to furthercomprise:determining the dielectric constant of the material to betested; and determining the elastic moduli of the material to be testedfrom the determined dielectric constant; and inputting the elasticmoduli into the processor.
 3. The method of claim 2 wherein the step ofdetermining the elastic moduli from the determined dielectric constantis further defined as comprising:providing sample composite materials,each sample composite material having material constituentssubstantially the same as the composite material to be tested and eachsample composite material having a different state of cure; determiningthe shear modulus and the Young's modulus and the dielectric constant ofeach of the sample composite materials to provide a correlation betweenthe Young's modulus and the dielectric constant and the correlationbetween the shear modulus and the dielectric constant of each of thesample materials which comprises a state of cure data base; determiningthe dielectric constant of the composite material to be tested; anddetermining the elastic moduli comprising the shear modulus and theYoung's modulus of the composite material to be tested using themeasured dielectric constant of the composite material to be tested andthe state of cure data base.
 4. The method of claim 1 wherein theacoustic waves are defined further as being ultrasonic waves.
 5. Themethod of claim 4 wherein the steps of propagating the two acousticwaves is defined further as propagating two acoustic waves through thecomposite material with one wave being a longitudinal wave and the otherwave being a shear wave.
 6. An apparatus for nondestructivelydetermining fiber volume fraction and resin porosity of a compositematerial constructed of at least two different constituent materialswherein the following parameters of the composite materials to be testedare known: density and layup sequence, the apparatus comprising:meansfor determining the elastic moduli of the composite material to betested; means for propagating two independent acoustic waves through thecomposite material; means for receiving the acoustic waves propagatedthrough the composite material and outputting the received acousticwaves in a digital format; means for determining the thickness of thecomposite material; and a processor receiving the two acoustic waves ina digital format and determining the velocity of each of the acousticwaves, V₁ and V₂, the processor having inputted therein the thickness ofthe composite material and the processor having inputted therein theelastic moduli of the composite material to be tested and the processorhaving inputted therein the known parameters of density and layupsequence, the processor determining the fiber volume fraction and resinporosity of the composite material using the velocities, V₁ and V₂, thethickness, the elastic moduli and the known parameters of density andlayup sequence.
 7. An apparatus for nondestructively determining fibervolume fraction and resin porosity of a composite material constructedof at least two different constituent materials wherein the followingparameters of the composite material to be tested are known: thickness,density and layup sequence, the apparatus comprising:means fordetermining the elastic moduli of the composite material to be tested;means for propagating two independent acoustic waves through thecomposite material; means for receiving the acoustic waves propagatedthrough the composite material and outputting the received acousticwaves in a digital format; means for determining the velocity of each ofthe two acoustic waves propagated through the composite material fromthe received and sensed acoustic waves propagated through the compositematerial and, the respective velocities being V₁ and V₂ ; and aprocessor receiving the two acoustic waves in a digital format anddetermining the velocities, V₁ and V₂, of the respective acoustic wavespropagated through the composite material, the processor having inputtedtherein the known parameters of thickness, density and layup sequenceand the processor having inputted therein the determined elastic moduli,the processor determining the fiber volume fraction and the resinporosity of the composite material using in the velocities, V₁ and V₂,the determined elastic moduli and the known parameters of thickness,density and layup sequence.
 8. A method, using a processor, fornondestructively determining fiber volume fraction and resin porosity ofa composite material constructed of at least two different materialswherein the following parameters of the composite material to be testedare known: density and layup sequence, the method comprising:determiningthe elastic moduli of the composite material to be tested; propagatingtwo independent acoustic waves from acoustic wave sources through thecomposite material at different positions over the surface area of thecomposite material to be tested; coupling the acoustic waves from theacoustic wave sources to the composite material with a fluid medium;receiving the acoustic waves propagated through the composite material;determining, in the processor, the velocity of each of the two acousticwaves propagated through the composite material from the receivedacoustic waves propagated through the composite material, the respectivevelocities being V₁ and V₂ ; determining the thickness of the compositematerial and putting the thickness into the processor; and determining,in the processor, the fiber volume fraction and resin porosity of thecomposite material using the determined elastic moduli, the velocities,V₁ and V₂, the thickness and the known parameters of density and layupsequence.
 9. The method of claim 8 wherein the step of determining theelastic moduli is defined further to comprise:determining the dielectricproperty of the material to be tested; and determining the elasticmoduli of the material to be tested from the determined dielectricproperty.
 10. The method of claim 8 wherein the step of determining theelastic moduli from the determined dielectric property is furtherdefined as comprising:providing sample composite materials, each samplecomposite material having material constituents substantially the sameas the composite material to be tested and each sample compositematerial having a different state of cure; determining the elasticmoduli (two independent moduli) and the dielectric property of each ofthe sample composite materials to provide a correlation between the twomoduli and the dielectric property which comprises a state of cure database; determining the dielectric property of the composite material tobe tested; and determining the elastic moduli (two independent moduli)of the composite material to be tested using the measured dielectricproperty of the composite material to be tested and the state of curedata base.
 11. The method of claim 8 wherein the acoustic waves aredefined further as being ultrasonic waves.
 12. The method of claim 11wherein the steps of propagating the two acoustic waves is definedfurther as propagating two acoustic waves through the composite materialwith one wave being a longitudinal wave and the other wave being a shearwave.
 13. An apparatus for nondestructively determining fiber volumefraction and resin porosity of a composite material constructed of atleast two different constituent materials wherein the followingparameters of the composite materials to be tested are known: densityand layup sequence, the apparatus comprising:means for determining theelastic moduli of the composite material to be tested; means forpropagating two independent acoustic waves through the compositematerial; means for providing a fluid medium for coupling the acousticwaves to the composite material; means for receiving the acoustic wavespropagated through the composite material and outputting the receivedacoustic waves in a digital format; means for determining the thicknessof the composite material; and a processor receiving the two acousticwaves in a digital format and determining the velocity of each of theacoustic waves, V₁ and V₂, the processor having inputted therein thethickness of the composite material and the processor having inputtedtherein the elastic moduli of the composite material to be tested andthe processor having inputted therein the known parameters of densityand layup sequence, the processor determining the fiber volume fractionand resin porosity of the composite material using the velocities, V₁and V₂, the thickness, the elastic moduli and the known parameters ofdensity and layup sequence.
 14. An apparatus for nondestructivelydetermining fiber volume fraction and resin porosity of a compositematerial constructed of at least two different constituent materialswherein the following parameters of the composite material to be testedare known: thickness, density and layup sequence, the apparatuscomprising:means for determining the elastic moduli of the compositematerial to be tested; means for propagating two independent acousticwaves through the composite material; means for providing a fluid mediumfor coupling the acoustic waves to the composite material; means forreceiving the acoustic waves propagated through the composite materialand outputting the received acoustic waves in a digital format; meansfor determining the velocity of the two acoustic waves propagatedthrough the composite material, the respective velocities being V₁ andV₂ ; and a processor receiving the two acoustic waves in a digitalformat and determining the velocities, V₁ and V₂, of the respectiveacoustic waves propagated through the composite material, the processorhaving inputted therein the known parameters of thickness, density andlayup sequence and the processor having inputted therein the determinedelastic moduli, the processor determining the fiber volume fraction andthe resin porosity of the composite material using in the velocities, V₁and V₂, the determined elastic moduli and the known parameters ofthickness, density and layup sequence.